The invention relates to high temperature superconductor current leads, and particularly to high temperature superconductor leads for carrying current to a device housed in a stratified fluid cryogenic system.
A major source of heat escaping into a cryogenic system typically occurs at the connection of the cryogenic system to the outside world. When the cryogenic system houses an operative superconducting device immersed in a cryogenic fluid, the "connection" is typically through the current leads, through which electrical energy passes to and from the device. The escape of heat through this connection is often referred to as "heat leak".
The chief mechanism of heat leak in a high temperature superconductor (HTS) current lead is generally conduction from the current lead to the cryogenic fluid. This is particularly true of leads made from composite high temperature superconductors. The rate of heat leak W of the superconductor lead can be approximated for design purposes as a function of thermal conductivity (k) averaged over the thermal length of the lead, cross-sectional area (A), temperature change (.DELTA.T) between the warm end HTS temperature and the cold end cryogenic fluid temperature, and thermal length (L) spanning the two temperatures, as described by the following relation: EQU W.apprxeq.kA.DELTA.T/L
As used herein, "thermal length" means the lead length over which the temperature gradient occurs, which can be less than the physical length of the lead. Heat leak from an HTS current lead can thus be reduced by decreasing k, A, and .DELTA.T, and by increasing L.